High performance circuit breaker with independent pole operation linkage and conical composite bushings

ABSTRACT

The invention provides a high performance circuit breaker with an independent pole operation linkage and conical composite bushings. A mechanical linkage for independently opening and closing a plurality of associated switches is provided by the invention. The linkage comprises a plurality of connecting rods that provide the initial driving force to open or close the switch, cranks for opening and closing the contacts of respective switches, linking elements which couple together the connecting rods and the cranks, and a number of lever assemblies having a bearing ring interfaced thereto. The bearing rings provide a supportive interface between two linking element but permit the linking elements to rotate independently from one another. The lever assemblies also provide an interface for the connecting rods and linking element. According to a method of the present invention, a connecting rod is actuated in response to a signal to cause the lever assembly to pivot. The linking rod interfaced with the connecting rod via the lever assembly is then rotated in response to the pivoting. A crank interfaced with the linking element is then pivoted in response to the rotation of the linking element thereby opening or closing a respective switch. The invention further provide a means and method for synchronously opening and closing the switches based on the current flowing into the switches. Preferably, conical composite bushings are provided in combination with the independent pole operation linkage in a high voltage circuit breaker according to the invention.

RELATED APPLICATION DATA

This application is a continuation-in-part of Ser. No. 08/196,590, filedon Feb. 11, 1994.

FIELD OF THE INVENTION

The present invention relates generally to electrical switching devices.More particularly, the invention relates to a synchronous independentpole operation linkage for use in a high voltage alternating currentcircuit breaker.

BACKGROUND OF THE INVENTION

A preferred application for the present invention is in high voltagealternating current (AC) three phase circuit breakers and reclosers, thelatter being a type of circuit breaker. Therefore, the background of theinvention is described below in connection with such devices. However,it should be noted that, except where they are expressly so limited, theclaims at the end of this specification are not intended to be limitedto applications of the invention in a high voltage three phase ACcircuit breaker.

A high voltage circuit breaker is a device used in the distribution ofthree phase electrical energy. When a sensor or protective relay detectsa fault or other system disturbance on the protected circuit, thecircuit breaker operates to physically separate current-carryingcontacts in each of the three phases by opening the circuit to preventthe continued flow of current. A recloser differs from a circuit breakerin that a circuit breaker opens a circuit and maintains the circuit inthe open position indefinitely, whereas a recloser may automaticallyopen and reclose the circuit several times in quick succession to allowa temporary fault to clear and thus, avoid taking the circuit out ofservice unnecessarily.

The major components of a circuit breaker or recloser include theinterrupters, which function to open and close one or more sets ofcurrent carrying contacts housed therein; the operating or drivingmechanism, which provides the energy necessary to open or close thecontacts; the arcing control mechanism and interrupting media, whichcreate an open condition in the protected circuit; one or more tanks forhousing the interrupters; and the bushings, which carry the high voltageelectrical energy from the protected circuit into and out of thetank(s). In addition, a mechanical linkage connects the interrupters andthe operating mechanism.

Circuit breakers may differ in the overall configuration of thesecomponents. However, the operation of most circuit breakers issubstantially the same regardless of their configurations. For example,a circuit breaker may include a single tank assembly which houses all ofthe interrupters. U.S. Pat. No. 4,442,329, Apr. 10, 1984, "Dead TankHousing for High Voltage Circuit Breaker Employing Puffer Interrupters,"discloses an example of the single tank configuration. Alternatively, aseparate tank for each interrupter may be provided in a multiple tankconfiguration. An example of a multiple tank configuration is depictedin FIG. 1.

As shown in FIG. 1, a prior art circuit breaker assembly 1 includesthree cylindrical metal tanks 3. The three cylindrical tanks 3 form acommon tank assembly 4 which is preferably filled with an inert,electrically insulating gas such as SF₆. The tank assembly 4 shown inFIG. 1 is referred to as a "dead tank" in that it is at groundpotential. Each tank 3 houses an interrupter (not shown in FIG. 1). Theoperation of an interrupter is described below. The interrupters areprovided with terminals which are connected to respective spaced bushinginsulators. The bushing insulators are shown as bushing insulators 5aand 6a for the first phase; 5b and 6b for the second phase; and 5c and6c for the third phase. Associated with each pole or phase is a currenttransformer 7.

SF₆ breaker bushings are an integral part of the breaker, bothelectrically and mechanically. They are not designed or used as generalpurpose apparatus bushings. SF₆ breaker bushings are designed to supportand insulate high voltage line connections and carry power into thegrounded tank of the circuit breaker.

In high voltage circuit breakers, the pairs of bushings for each phaseare often mounted so that their ends have a greater spacing than theirbases to avoid breakdown between the exposed conductive ends of thebushings. One means for achieving the desired spacing has been to useconical bushings such that the terminal ends of the bushings havesmaller diameters than their respective bases. For example, FIG. 1Ashows a high voltage circuit breaker with conical bushings 90a-c and92a-c. The conical bushings are angled away from each other to providean adequate air gap (AG) between their ends so that in the event of aflashover or significant current leakage, the resulting breakdown isgrounded in the dead tank. Therefore, it is desirable that the spacingbetween the terminal portions of the bushings, i.e., air gap, be greaterthan the length of the bushing. As circuit breakers become more compact,the size and spacing of the bushings become a critical design feature ofthe circuit breaker.

A longitudinal cross section of a typical conical bushing is shown inFIG. 1B. A high voltage conductor 100 is surrounded by an insulator 101with weather sheds 102. The conductor 100 is electrically coupledbetween an interrupter and the protected circuit. The insulator 101 ofthe SF₆ breaker bushing is shaped and sized to accommodate an internalgrading shield 103 which optimizes dielectric strength (internally andexternally). The shield 103 is uniquely shaped to grade the voltagefield in the air along the exterior length of the bushing as well asinside where the bushing conductor 100 enters the grounded tank. Thebushing conductor 100, running through the hollow, SF₆ -filled insulator101, creates a radial stress through the bushing. This stress is higherat the entrance to the grounded tank. Therefore, the shield 103 reducesthe stress on the insulator to improve the reliability of the bushing.The weather sheds 102 on the external surface of the bushing resist theeffects of rain and surface dirt to maintain good dielectric conditions.

Traditionally, bushing insulators have been made from porcelain or acast epoxy. Typically, the weather sheds are designed so that waterrolls off the sheds keeping the underside of the sheds substantiallydry. However a significant portion of the insulator surface can becomewet or degraded by environmental pollution. The resulting weakening ofthe dielectric can cause leakage and flashover conditions.

An additional drawback of porcelain or cast epoxy bushings is that theyare relatively brittle and, therefore, subject to damage from externalcondition that can cause them to shatter so that the SF₆ containedtherein explodes. To provide an optimal insulator and a safe andreliable housing for the bushing conductor, the porcelain and cast epoxyinsulators are produced with a relatively thick wall (i.e., about 1inch). The increased thickness further narrows the air gap, increasesthe weight of the bushings, and increases the cost of the bushings.

Therefore, a composite bushing has been developed that provides thefollowing advantages over traditional bushings: non-brittle behavior,reduced weight and wall thickness, pollution resistance, and improvedwet electrical capability. A longitudinal cross section of a compositebushing is shown in FIG. 1C. Composite bushings insulators are made upof a fiberglass reinforced tube 110 protected by a silicone rubberhousing 112. These bushings have a straight cylindrical composite tubewith aluminum end flanges 114 and 116 and room temperature, vulcanized(RTV) silicone rubber weather sheds 120. The RTV silicone rubber has ahydrophobic surface due to oil films that naturally form on the rubbersurface.

The composite bushings are produced by using an injection moldingtechnique in which the a single mold forms a single section of thehousing 112 at a time. This process is both time consuming andrelatively inefficient in that each section of the housing must also bemolded together to form the completed bushing housing. Since thesilicone rubber housing is formed from a injection molding process, aspecially designed mold would be required to produce the desired conicalshape. For many high voltage breakers that require very large bushings,such molds are impractical.

A process for molding rubber using a traveling mold has beencommercially exploited. Essentially, the traveling mold is capable offorming plastic or rubber on substantially any shape in a continuousprocess. Therefore, to improve the performance and reduce the size andweight of high voltage circuit breakers there is a need to design aconical composite bushing that has a housing which can be formed usingsuch a traveling mold.

Two other important elements of a high voltage circuit breaker are theoperating mechanism and a mechanical linkage. The operating mechanismthat provides the necessary operating forces for opening and closing theinterrupter contacts is contained within an operating mechanism housing9 shown in FIGS. 1 and 1A. The operating mechanism is mechanicallycoupled to each of the interrupters via a linkage 8.

A cross section of an interrupter 10 is shown in FIGS. 2A-C. Theinterrupter provides two sets of contacts, the arcing contacts 12 and 14and the main contacts 15 and 19. Arcing contacts 12 and main contacts 19are movable, as described in more detail below, to either close thecircuit with respective contacts 14 and 15 or to open the circuit. FIG.2A shows a cross sectional view of the interrupter with its contactsclosed, whereas FIG. 2C shows a cross section of the interrupter withthe contacts open.

The arcing contacts 12 and 19 of high voltage circuit breakerinterrupters are subject to arcing or corona discharge when they areopened or closed, respectively. As shown in FIG. 2B, an arc 16 is formedbetween arcing contacts 12 and 14 as they are moved apart. Such arcingcan cause the contacts to erode and perhaps to disintegrate over time.Therefore, a known practice (used in a "puffer" interrupter) is to filla cavity of the interrupter with an inert, electrically insulating gasthat quenches the arc 16. As shown in FIG. 2B, the gas is compressed bypiston 17 and a jet or nozzle 18 is positioned so that, at the propermoment, a blast of the compressed gas is directed toward the location ofthe arc in order to extinguish it. Once an arc has formed, it isextremely difficult to extinguish it until the arc current issubstantially reduced. Once the arc is extinguished as shown in FIG. 2C,the protected circuit is opened thereby preventing current flow.

Typically a bank of shunt capacitors is coupled between the arcingcontacts to control the arcing by equalizing the voltages at therespective breaks in a multi-interrupting point type circuit breaker,i.e., one with more than one set of contacts. A capacitor coupledbetween contacts may also be used in a single-break circuit breaker. Thebank of shunt capacitors is typically arranged within a dead tank tosurround an arc-extinguishing chamber therein. It is further known tocontrol arcing utilizing pre-insertion or closing resistors, asdisclosed in U.S. Pat. No. 5,245,145, Sep. 14, 1993, "Modular ClosingResistor" (assigned to ABB Power T&D Company Inc.).

Voltage and current transients generated during the energization ofshunt capacitor banks have become an increasing concern for the electricutility industry in terms of power quality for voltage-sensitive loadsand excessive stresses on power system equipment. For example, moderndigital equipment requires a stable source of power. Moreover,computers, microwave ovens and other electronic appliances are prone tofailures resulting from such transients. Even minor transients can causethe power waveform to skew, rendering these electrical devicesinoperative. Therefore, utilities have set objectives to reduce theoccurrence of transients and to provide a stable power waveform.

Conventional solutions for reducing the transients resulting from shuntcapacitor energization include circuit breaker pre-insertion devices,for example, resistors or inductors, and fixed devices such as currentlimiting reactors. While these solutions provide varying degrees ofmitigation for capacitor bank energization transients, they result inadded equipment, added cost, and can result in added reliabilityconcerns.

The maximum shunt capacitor bank energization transients are associatedwith closing the circuit breaker at the peak of the system voltagewaveform, i.e., where the greatest difference exists between the busvoltage, which will be at its maximum, and the capacitor bank voltage,which will be at a zero level. Where the closings are not synchronizedwith respect to the system voltage, the probability for obtaining themaximum energization transients is high. One solution to this problem isto add timing accuracy to synchronously close the circuit breaker at theinstant the system voltage is substantially zero. In this way, thevoltages on both sides of the circuit breaker at the instant of closurewould be nearly equal, allowing for an effectively "transient-free"energization.

While the concept of synchronous or zero-crossing closing is a simpleone, a cost-effective solution has been difficult to achieve, primarilydue to the high cost of providing the required timing accuracy in amechanical system. U.S. Pat. No. 4,306,263, Dec. 15, 1981, entitled"Synchronous Closing System and Latch Therefor," discloses a synchronousclosing system wherein the circuit breaker main contacts close withinabout 1 millisecond of a zero crossing by inhibiting the hydraulicpressure utilized to close the interrupter contacts using a latchcontrolled mechanism. However, this synchronous closing system isincapable of providing synchronization for each phase or poleindividually. Thus, while one phase may be closed synchronously,avoiding transients in that phase of the circuit, harmful transients maybe produced by closing the contacts in one or both of the other phases.

One solution might be to utilize three separate operating mechanisms andcorresponding linkages to synchronously control the operation of eachpole individually. U.S. Pat. No. 4,417,111, Nov. 22, 1983, entitled"Three-Phase Combined Type Circuit Breaker," discloses a circuit breakerhaving a separate operating mechanism and associated linkage for each ofthe three phases or poles. However the use of three separate operatingmechanisms and associated linkages is expensive and increases theoverall size and complexity of the circuit breaker.

U.S. Pat. No. 4,814,560, Mar. 21, 1989, "High Voltage Circuit Breaker"(assigned to Asea Brown Boveri AB, Vasteras, Sweden) discloses a devicefor synchronously closing and opening a three-phase high voltage circuitbreaker so that a time shift between the instants of contact in thedifferent phases can be brought about mechanically by a suitable choiceof arms and links in the mechanical linkage. This linkage uses an apriori knowledge of the time required to close and open the interruptercontacts in each of the three phases. The time differences can beaccounted for by an appropriate design of the mechanical linkage.However, such a linkage cannot support dynamic monitoring of thezero-crossings for each phase to achieve independent synchronization.Moreover, the mechanical linkage disclosed would require mechanicaladjustments over time to account for variations in the circuit breakerperformance and operating conditions which often change over time.

A dependent pole operating mechanism has been used in circuit breakersto generate the initial driving forces required to open and close theinterrupter contacts. Dependent pole operation refers to the limitedcapability of the operating mechanism to close or open all three phasesof the circuit simultaneously. A prior art example of a dependent polemechanism and mechanical linkage implemented in a three-phase circuitbreaker is shown in FIG. 3. As shown in FIG. 3, operating mechanism 20provides a single connecting rod 22. Connecting rod 22 is interfacedwith linking element 26 via lever 24. Linking elements 25 and 26preferably form a single linking shaft linking together the terminalportions of each of the three interrupters (not shown in FIG. 3). Inoperation, the connecting rod 22 is driven up or down thereby pivotinglever 24. As lever 24 pivots, the linking elements 25 and 26 rotate. Thelinking elements are preferably coupled to bell cranks provided in theterminal portion of the interrupters (not shown in FIG. 3) which pivotin response to the rotation of the linking elements to open and closethe contacts of the interrupters. It should be understood that each ofthe interrupters housed in tanks 3 will open and close simultaneously inresponse to the movement of connecting rod 22.

Recently an independent pole operating mechanism has been developedwhich provides an individually controlled driving force for opening andclosing each phase of the circuit breaker independently. By utilizingthe independent pole operating mechanism, each phase can be dynamicallyand synchronously switched individually. Thus there is a need to providea mechanical linkage to operate effectively with the independent poleoperating mechanism. To eliminate the necessity of redesigning theentire circuit breaker to implement the new independent pole operatingmechanism, it is desirable to cost-effectively adapt existing circuitbreaker linkages, such as linkage 8 shown in FIG. 1. Moreover, themechanical linkage for use with the independent pole operating mechanismshould not increase the size of the circuit breaker, or require complexassembly or maintenance steps to ensure that the circuit breakerfunctions properly.

SUMMARY OF THE INVENTION

The present invention fulfills these needs by providing a mechanicallinkage for independently opening and closing a plurality of associatedswitches. According to the invention the mechanical linkage comprises aplurality of linking elements, and a decoupling means for rotationallydecoupling and supporting the linking elements at an interface betweenthe linking elements. A plurality of connecting rods extending from adriving mechanism are operatively coupled to the linking elements. Thedriving mechanism actuates connecting rods to cause the linking elementsto open and close the plurality of switches. In a preferred embodiment,the decoupling means comprises a number of lever assemblies forpivotally coupling one of the connecting rods with one of the linkingelements to operatively rotate the linking element, and a number ofbearing means for providing a supporting link at an interface betweentwo of the linking elements so that the two linking elements arerotationally decoupled from one another at the interface therebetween.

A three-phase circuit breaker is also provided according to theinvention for opening and closing a circuit connected thereto. In apreferred embodiment, the three-phase circuit breaker comprises a numberof interrupting means; a linking mechanism comprising a plurality oflinking elements, and a decoupling means for rotationally decoupling andsupporting the linking elements at an interface between the linkingelements; and a driving means having at least two connecting rodsmechanically interfaced with the linking elements so that the linkingelements rotate independently with respect to each other to open andclose associated phases of the circuit. The linking mechanism preferablycomprises at least two lever assemblies for mechanically interfacing oneconnecting rod with one linking element; and at least two bearing meanscapable of providing a supportive interface for two of the linkingelements. The driving means is preferably an operating mechanism havinga three-phase independent pole operation capability. In a morepreferable embodiment, the circuit breaker is opened and closedsynchronously with AC current flowing into the interrupters.

A method for controlling the opening and closing of a plurality ofassociated switches is also provided to fulfill the above-identifiedneeds. The inventive method comprises the steps of receiving a signal toinitiate the switching of one of the switches to open or to close;actuating a connecting rod interfaced with one linking element togenerate an independent movement of the linking element with respect toother linking elements in response to the signal; and switching theswitch open or closed in response to the independent movement of thelinking element. In a preferred embodiment, a crank which is operativelycoupled between a linking element and a switch is pivoted in response tothe independent movement of the linking element so that the switch isswitched open or closed in accordance with the received signal. In afurther preferred embodiment, the interface between the connecting rodand the linking element is provided by a lever. In this preferredembodiment, the lever is pivoted in response to actuating the connectingrod and the linking element is rotated in response to the pivoting. Theinterface between the connecting rod and the linking element ispreferably periodically adjusted to ensure that the lever pivotssufficiently to open and close the switch.

A three phase independent pole operation circuit breaker comprising: anumber of switches where each switch is associated with one of thephases of a protected circuit and operatively interfaced with thecircuit to form a high voltage interface; an independent pole operationlinkage mechanically connected to each switch and being capable ofindependently opening or closing each switch based on a condition of theprotected circuit; and at least one composite bushing interfaced withthe protected circuit and interfaced with each switch for insulating thehigh voltage interface. In a preferred embodiment, the composite bushinghas a conical shape with a plurality of weather sheds forming a helixthereon.

in a more preferred embodiment, composite bushing comprises: a hollowconical tube for containing a gas within the bushing; and an insulativehousing having a plurality of weather sheds and substantially coveringthe hollow conical tube. The hollow conical tube is preferably made of afiber reinforced epoxy material and the housing is preferably made ofsilicone rubber.

In another preferred embodiment, the switches are opened and closedsynchronously with AC current flowing in each of the phases of theprotected circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood and its numerous objectsand advantages will become apparent by reference to the followingdetailed description of preferred embodiments when taken in conjunctionwith the following drawings, in which:

FIG. 1 is a diagram of a prior art multiple tank high voltage circuitbreaker;

FIG. 1A is a diagram of a high voltage circuit breaker having conicalbushings arranged in an angular configuration;

FIG. 1B is a longitudinal cross section of a conical bushing accordingto the prior art;

FIG. 1C is a longitudinal partial cross section of a cylindricalcomposite bushing;

FIG. 2A is a cross sectional view of a prior art interrupter with itscontacts closed;

FIG. 2B is a cross sectional view of a prior art interrupter with an arcformed between its arcing contacts;

FIG. 2C is a cross sectional view of a prior art interrupter with itscontacts open;

FIG. 3 is a diagram of a prior art a dependent pole operation linkageaccording to the present invention;

FIG. 4 is a diagram of an independent pole operation linkage inaccordance with the invention;

FIG. 5 is a cross sectional view of an interrupter with a bell crankcoupling; and

FIG. 6 is an enlarged view with a partial cross sectional view of thelever assembly and bearing ring according to the present invention,

FIG. 7 is longitudinal cross-section of a conical composite bushingaccording to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A diagram of a linkage for use in a three-phase circuit breakeraccording to the invention is shown FIG. 4. The operating mechanism 30preferably provides three independently operated connecting rods 32, 33and 34. The linkage comprises four link elements 36, 37, 38 and 39. Itshould be understood that in a preferred embodiment, link elements 37and 38 form a single linking shaft. Two lever assemblies 40 and 41provide mechanical interfaces for coupling connecting rod 32 with linkelement 39 and for coupling connecting rod 33 with link element 37,respectively. Lever assemblies 40 and 41 also provide a mechanicalinterface to bearing rings 45 and 46 respectively. Connecting rod 34 ismechanically interfaced with link element 36 via lever 43 which does nothave a mechanical interface to a bearing ring. Bearing rings 45 and 46provide a supportive link between link elements 38 and 39 and betweenlink elements 36 and 37, respectively. These supportive links permiteach of the linking elements to rotate independently from each other, asdescribed below.

Each link element is coupled to a bell crank (not shown in FIG. 4)located within the terminal portion 42 of the interrupter shown withintank 3 in FIG. 4. FIG. 5 shows an axial cross sectional view of aninterrupter. A bell crank 50 is shown in the terminal portion of theinterrupter. When the bell crank 50 is pivoted in the directionindicated by the dash-and-dot lines, the insulating rod 52 is pivotedinto the guide tube 54 causing a piston member 53 to close theinterrupter's contacts, i.e. movable contacts 12 and 19 so that theyengage fixed contacts 14 and 15, respectively. The link elements 36 and39 (FIG. 4) are preferably coupled to bell crank shafts 48 and 49respectively with a clamp or other suitable means. Link elements 37 and38 are preferably bell crank shafts coupled to a common bell crank.

To close one pole or phase of the protected circuit, the operatingmechanism 30 drives the appropriate connecting rod 32, 33, or 34 upward.For instance, if it were desired to close the contacts of theinterrupter located within the center tank 3 shown in FIG. 4, connectingrod 33 would be extended upward. The upward motion of connecting rod 33pivotally rotates lever assembly 41 causing link element 37 to rotateabout its axis in a clockwise direction as shown. Link elements 37 and38 are coupled via splining or an equivalent method to bell crank 50(FIG. 5). Bell crank 50 preferably has two lateral faces (one is shownin FIG. 5) so that link element 37 is coupled to one lateral face andlink element 38 is coupled to the other lateral face. Thus, as linkelement 37 rotates, bell crank 50 pivots along the dash-and-dot line inFIG. 5 to close the interrupter's contacts.

Typically a utility which maintains the protected circuit monitors faultconditions on the circuit. If a fault condition is detected a controlsignal is output to the circuit breaker to cause the operating mechanismto close the appropriate contacts. Manual switches may also be providedfor generating similar signals to initiate the opening or closing of thecontacts. The operating mechanism 30 in response to such signalsreleases the appropriate connecting rod 32, 33 or 34. For instance, toopen the phase of the circuit associated with the center interruptershown in FIG. 4, connecting rod 33 is pulled in a downward motion. Leverassembly 41 pivots causing link element 37 to rotate in a counterclockwise direction. The counterclockwise rotation of link element 37 inturn causes bell crank 50 to pivot away from the interrupter's contactsalong the dash-and-dot line to open up the contacts.

It should be understood that the size of the connecting rods, linkingelements, and bell cranks, as well as the pivot angle of the bell crankshould be considered to determine the angle through which the leverassemblies 40 and 41 and lever 43 must rotate to properly open and closethe contacts. One advantage of the linkage design according to theinvention is that the linkage is easily assembled for independent phaseoperation by adjusting the joints (the lever assembly/linking elementinterfaces and the lever 43 interface with link element 36 define thejoints) to achieve the required angle of rotation. Thus only oneadjustment is required for each operational phase of the circuit. Otherlinkages, e.g., linear linkages, can require as many as five adjustmentsper phase.

FIG. 6 shows an enlarged view of lever assembly 40 with the portionbelow the dash-and-dot line showing a cross section of the lever 65 andbearing 45. As shown in the figure, link element 39 can be insertedthrough an aperture 62 in lever 65 in the direction indicated by arrow59. The link element 39 may be mechanically coupled to lever 65 via anysuitable method such as splining, pinning, bolting or the like. Lever 65also provides the outer shell 58 of the bearing ring (reference numeral45 in FIG. 4). Therefore, lever 65 also provides a hollow opening 63 inwhich the individual bearings 67 can be inserted to form the bearingring with an inner aperture 64. Link element 38 can be inserted throughaperture 64 in the direction of arrow 60. Thus the bearing ring 45 formsa supportive link between link elements 38 and 39. In a more preferredembodiment, a sleeve 56 lines the aperture 64 and serves as a spacer sothat standard sized bearings may be used in the lever assembly. A springpin 57 or the equivalent thereof may be projected through the leverassembly 40 as shown to secure the bearings 67 within the hollow opening63.

The linkage according to the present invention provides numerousadvantages for independent phase operation in a circuit breaker.Significantly the standard size linkage as shown for instance in FIG. 3can be adapted rather than completely redesigned to accommodateindependent pole operation. For example, lever assemblies and bearingrings can be added to rotationally decouple the linking elements 25 and26 of the dependent pole linkage 8 to provide an independent phaseoperation capability. It should, therefore, be evident that the linkageaccording to the present invention is substantially the same size as thedependent pole linkage so that existing circuit breaker hardware can beused with the independent pole operation linkage. Moreover, the linkageaccording to the present invention does not increase the size of thecircuit breaker to achieve independent pole operation.

FIG. 7 is a longitudinal cross section of a conical composite bushingaccording to the present invention. A conical fiber reinforced tube(FRP) 120 surrounding the bushing conductor 121 is formed from an epoxyresin or polyester material which has been reinforced with a strongfibrous material such as fiberglass, polyesters, aramids, or cloththreads. The traveling mold described above is used to form aninsulative housing 122 having weather sheds 124 of silicone rubber or asimilar rubber material such as ethylene propylene on the surface of theconical FRP tube 120. The weather sheds 124 preferably form a helixalong the bushing surface. A grading shield 126 is attached to the innersurface of the FRP tube 120 and mounting flange 128 as shown in FIG. 7.A top flange 130 secures an 0-ring 132 to the bushing. A line terminal134 from the protected circuit is electrically interfaced to the bushingconductor 121 at the 0-ring 132.

The performance of an independent pole operation circuit breaker can befurther improved by replacing conventional bushings with the conicalcomposite bushings. Not only do the composite bushings repel pollutionand water, resist shattering or exploding, reduce size and weight of thebreakers, but they can additionally improve the breaker's overallreliability in other ways. For instance, if any of the bushings fail inconventional dependent pole operating circuit breaker, the entirecircuit breaker must be shut down. However, in a independent poleoperating circuit breaker only the phase associated with a faultybushing need be opened. When conical composite bushings are utilizedwith an independent pole operation linkage the need to shut down any ofthe phases is further reduced by the superior attributes of the conicalcomposite bushings.

Moreover, in conventional synchronous closing breakers, leakage in anyone of the bushings can result in altering the timing of the closing oropening of the circuit. In an independent pole synchronous closingcircuit breaker, leakage in one bushing should not affect the remainingphase. However, minimizing leakage using the conical composite bushingsalso improves the reliability of an independent pole synchronous closingcircuit breaker since even small leaks can affect the timing accuracywhen opening or closing any of the phases of the protected circuit.

While the invention has been described and illustrated with reference tospecific embodiments, those skilled in the art will recognize thatmodification and variations may be made without departing from theprinciples of the invention as described herein above and set forth inthe following claims.

What is claimed:
 1. A three phase independent pole operation circuitbreaker comprising:a plurality of switches, each switch being associatedwith one of the phases of a protected circuit and operatively interfacedwith such protected circuit to form a high voltage interface, each saidswitch having a set of contacts for opening and closing said associatedphase of said protected circuit; an independent pole operation linkagemechanically connected to each switch and being capable of independentlyopening or closing each switch based on a condition of the protectedcircuit said linkage comprising a plurality of independent linkingelements, each linking element being operatively coupled to one set ofcontacts to open and close said associated phase; and means forrotationally supporting each of said linking elements; a driving meanshaving at least two connecting rods, each connecting rod beingmechanically interfaced with at least one linking element so that saidlinking elements rotate independently with respect to each other therebyindependently opening and closing said associated phases of saidprotected circuit; and at least one composite bushing interfaced withsaid protected circuit and interfaced with each said switch forinsulating said high voltage interface.
 2. The circuit breaker of claim1, wherein said composite bushing has a conical shape.
 3. The circuitbreaker of claim 1, wherein said composite bushing has a plurality ofweather sheds formed thereon.
 4. The circuit breaker of claim 3, whereinsaid weather sheds form a helix.
 5. The circuit breaker of claim 1,wherein said composite bushing comprises:a hollow conical tube forcontaining a gas within the bushing; and an insulative housing having aplurality of weather sheds and substantially covering said hollowconical tube.
 6. The circuit breaker of claim 5, wherein said hollowconical tube is made of a fiber reinforced epoxy material.
 7. Thecircuit breaker of claim 5, wherein said housing is made of siliconerubber.
 8. The circuit breaker of claim 1, wherein AC current flows ineach phase of said protected circuit and said switches are opened andclosed synchronously with said AC current.
 9. The circuit breaker ofclaim 1, wherein two composite bushings are provided for each phase ofthe protected circuit.
 10. In a three-phase circuit breaker comprisingthree interrupters for operatively opening and closing said circuitbreaker, each interrupter being associated with one of said threephases; a driving mechanism for generating an initial force to open andclose said circuit breaker; and a mechanical linkage for operativelycoupling together said interrupters and said driving mechanism, theimprovement comprising:a decoupling and support means for(i) decouplingsaid mechanical linkage to form a plurality of independent linkingelements therein; (ii) supporting said linking elements; and, (iii)permitting each phase of said circuit breaker to be independently openedand closed, said decoupling and support means comprising a bearingthrough which two of said linking elements form a rotationally decoupledlink; and, at least one composite bushing interfaced with each saidinterrupter for interfacing with a circuit to be protected.